Therapeutic agents in nanoparticles, such as liposomes, may passively reach target organs, tissues, cell, or microenvironments such as tumor microenvironments. Tumor-associated macrophages (TAMs) play a role in tumor growth and metastasis, actively maintaining a immune-suppressive state in the tumor microenvironment. TAMs also appear to contribute to resistance to various therapeutic agents and to promote tumor cell growth and metastasis.